Prolaction is a hormone that may share properties with two other polypeptide hormones, growth hormone and chorionic somotomammotrophin. Although prolactin is purported to have numerous functions in lower animals, a function of this hormone in non-lactating mammals has yet to be found. Various studies in animals and in humans have not proven conclusively that prolactin has anti-insulin effects. The studies that show a diabetogenic effect are weakened by the use of prolactin preparations which are contaminated with growth hormone and probably other hormones as well. In clinical studies it is also diffiult to separate the effects of prolactin from influences of other trophic hormones. This study will look at carbohydrate and fat metabolism in animals with endogenous hyperprolactinemia due to pituitary implants placed under the kidney capsule. Through the use of column chromatography, radiommunoassay, and radioreceptor techniques, the rat model will be extensively studied to confirm its reported effectiveness as a system for study of excess prolactin ecretion. The effects of endogenous hyperprolactinemia, in the absence of other pituitary abnormalities, on serum glucose and insulin, fat cell size and number, growth, and susceptibility to streptozotocin-induced diabetes mellitus will be investigated.